Interaction of Oligocene–Miocene Deep-water Depositional Systems with Actively Evolving Structures

Abstract

The Angola continental margin has undergone a complex history of gravity-driven deformation. The interaction of actively evolving structures and depositional systems is a primary control on Oligocene–Miocene reservoir distribution and architecture. Structurally driven changes in sea-floor gradient can lead to rapid lateral changes in reservoir distribution and geometry. Understanding this complexity as the Lower Congo Basin (LCB) evolved through time is an important focus of exploration efforts in the basin. Deep-water clastic reservoirs in the LCB range from the Rupelian to the Messinian and were deposited on the LCB slope; they are generally organized into confined and weakly confined slope-channel systems. As the Angola margin evolved, sediment gravity flows were diverted through a maze of active structures, forming multicycle stacked channel systems. In the distal reaches of the paleoslope where the paleobathymetry was more subdued, the sands form single-cycle, digitate, anastomosing to distributary bodies. Sheet geometries are mostly absent because of the continued presence of a low-slope regional gradient through the extent of three-dimensional (3-D) coverage. Considerable lateral variability in structural style exists along the Angola continental margin, where updip extension and translation are accommodated by downslope contraction and extrusion of salt. Because Oligocene–Miocene strata are syntectonic deposits, the active structures strongly influenced deposition and, therefore, the present-day distribution of reservoir sands on the LCB slope. Significant changes occur in the interaction of the depositional system and structures during the early Miocene. From the Rupelian to the early Burdigalian, proximal extensional normal faults had minimal impact on sediment distribution. Downdip, in the contractional domain, the interaction was more subtle and episodic. The style of interaction and depositional system response depend on the rate of local sedimentation compared with the rate of uplift for each structure. This can vary significantly even with structures in close proximity. From the late Burdigalian until the Messinian, active structures in both the extensional and contractional domains controlled the distribution of coarse clastic sediments. In the central LCB, sediment capture in the updip grabens created a coarse clastic sediment shadow downdip. In the contractional domain, the channel systems were deflected between and around the active structures.